Abstract: An exhaust gas treatment system and method, primarily for a motor vehicle having an internal combustion engine. An expansion chamber is connected in a straight line to a first straight line portion of an exhaust conduit from the engine, with a second portion of the exhaust conduit extending in a straight line from the expansion chamber. The expansion chamber has a cross-sectional area 110% to 200% as large as the cross-sectional area of the exhaust conduit. First and second series of openings are provided in the expansion chamber substantially 180.degree. apart along the circumference of the expansion chamber, with the expansion chamber being mounted so that a plane containing the first and second series of openings is generally vertical. Pick-off conduits return carbon monoxide, hydrocarbon gases, and water vapors separated through the openings of the expansion chamber to the engine, the returning gases being filtered to at least remove the excess liquid therein.